Refactor key manager hasher

[AaronFeickert]

The hasher used by the key manager is instantiated using a wrapper function that, while generic in functionality, indicates use for generating a MAC.
This is not accurate; the hasher is used for secret key derivation, stream cipher nonces, MAC generation, and PBKDF salts.
This isn't unsafe, but could be refactored for clarity to avoid any confusion in the future.

[AaronFeickert]

Note that this wouldn't need to be a breaking change.